TMR (tunneling magnetoresistance) sensors are a new magnetoresistive sensing technology that is beginning to find use in industrial applications. This sensor technology uses tunneling magnetoresistive multilayer thin film materials for sensing the magnetic field, and the main features are: the magnetic multilayer film produces a big change in resistance in response to a change in magnitude or direction of the external magnetic field. Compared to other practical applications utilizing the AMR effect (anisotropic magnetoresistance) of the GMR effect (giant magnetoresistance), the rate of change of resistance is much greater, and additionally compared to the Hall Effect it has much better temperature stability.
Although both GMR and TMR are compatible with standard semiconductor manufacturing processes, high sensitivity GMR or TMR sensors have yet not begun low cost mass production. The production yield of GMR and TMR magnetoresistive sensors depends on the achievable offset value, and this is difficult when forming a bridge utilizing GMR or TMR magnetoresistive elements. In order to realize low cost mass production of GMR or TMR sensors, and also to produce single-chip GMR or TMR sensors, there presently are three methods that are used to achieve high sensitivity GMR or TMR sensors.
(1) Through the use of a two film process or local laser annealing, it is possible to set the magnetization of the pinned layers of the different bridge arms in different directions, in order to realize a single-chip sensor bridge. In the two film process, the two respective TMR pinned layers are set in opposite directions; it makes the process complicated, because it requires a second annealing that affects the layer that was deposited first, the makes the matching of the two films less consistent, which affects the overall performance of the sensor. When using local laser annealing to locally flip the pinned layer magnetization, the same film is used for both arms but it is locally annealed in a strong magnetic field, to make the two adjacent pinning layers have opposite pinned layer magnetization direction, in order to achieve a single-chip magnetic field sensor bridge. Unfortunately, this method requires custom equipment, which is expensive to develop, and the local laser annealing process is slow.
(2) The free layer's magnetization direction can be tilted. Each sensor arm has the same pinned layer magnetization direction, but the free layer of adjacent arms may have different magnetization directions, wherein the angle of the free layer magnetization with respect to the pinned layer magnetization has the same magnitude but different polarity for the different arms. Unfortunately, this method leads to smaller dynamic range of the sensor response, reducing the operating range.
(3) using magnetic shielding to provide flux concentrators in referenced bridge sensors, in the present art this method results in large spatial separation between the reference and sense arms making it difficult to control offset, large die size, and high cost.
FIG. 1 shows the present common implementation of prior art single-chip magnetic field sensor bridges. The structure includes a silicon substrate 1, shielding structures 2, sense elements 3, reference elements 4, a gap 5, four wire bonding pads 7-10 used for input and output, one is for power Vbias, one is for grounding GND, and two are for output voltage V+, V−, and the sensor detects the field along a sensing axis 100. The reference elements are located under the shielding structures 2, and the sense elements 3 are located in the gap 5 between the shielding structures 2, wherein the shielding structures have a rectangular shape. The sense elements 3 are interconnected to form a sense arm, and the reference elements 4 are interconnected to form a reference arm, the sense arm 3 and the reference arm 4 are composed of GMR sensing elements. The silicon substrate 1 has its longest dimension parallel to the sensing axis 100, the reference elements 4 and sensing elements 3 are spatially located far apart, leading to a long distance between the sense arms and reference arms; there is only one gap 5, this arrangement is spatially inefficient, and it makes the chip size large, typically this kind of design leads to a chip size of at least 2 mm×0.5 mm. Also, because the spacing between the sense arms and reference arms is large, the bridge is not well balanced, and the reference arms and sensing arms can have different temperatures, making thermal compensation of the bridge less effective. In addition, the rectangular shield structures 2, are easily saturated in moderate magnetic fields, leading to nonuniform magnetic field distribution between the center and the edge, and it can produce magnetic hysteresis, all of these issues reduce the linearity of the sensor.